Uric acid is the final product of purine metabolism in human. In many mammals, unlike human, uric acid is further broken down by urate oxidase (uricase) in the liver into allantoin, which is excreted through the kidney. In human, main pathway of uric acid excretion is the kidney, wherein approximately two thirds of uric acid is excreted in urine. The remaining is excreted in feces. When an excessive production or decreased excretion of uric acid occurs, that causes hyperuricemia. Hyperuricemia is classified into a uric acid overproduction type, a uric acid underexcretion type and a mixed type thereof. This classification of hyperuricemia is clinically important. Aiming for reducing adverse effects of therapeutic agents, therapeutic agents are chosen according to each class (for example, see Non-patent reference 1).
In hyperuricemia with a uric acid overproduction type, urinary excretion of uric acid increases, and when the urinary excretion of uric acid further increases by using of a uricosuric drug, the complication of urinary calculi is possibly developed. Therefore, in principle, allopurinol, a uric acid production inhibitor (or sometimes called a uric acid synthesis inhibitor, hereinafter referred to as “a uric acid production inhibitor”), is used in a uric acid overproduction type.
Uric acid is produced from purine bodies, which are derived from diet and synthesized endogenously, finally by oxidizing xanthine by xanthine oxidase. Allopurinol is developed as a xanthine oxidase inhibitor and an only uric acid production inhibitor used in medical practice. While allopurinol, however, is reported being effective in hyperuricemia and various diseases caused by the same, severe adverse effects such as poisoning syndrome (hypersensitivity angiitis), Stevens-Johnson syndrome, exfoliative dermatitis, aplastic anemia, liver dysfunction and the like have been also reported (for example, see Non-patent reference 2). As one of the causes, it has been pointed out that allopurinol has a nucleic acid-like structure and inhibits a pathway of pyrimidine metabolism (for example, see Non-patent reference 3).
On the other hand, in hyperuricemia with a uric acid underexcretion type, uric acid excretion decreases. It has been reported that when allopurinol, which is metabolized into oxypurinol to be excreted through the kidney by the same mechanism to uric acid, is used, the excretion of oxypurinol also decreases and that increases the incidence of liver disorders (for example, see Non-patent reference 4). Therefore, in principle, uricosuric drugs such as probenecid, benzbromarone and the like are used in a uric acid underexcretion type. These uricosuric drugs, however, also exert adverse effects such as gastrointestinal disorders, urinary calculi or the like. Particularly, benzbromarone is known as possibly causing fulminant hepatitis in the case of idiosyncratic patients (for example, see Non-patent references 5 and 6).
Thus, it is said that both of the existing uric acid production inhibitor and uricosuric drug have usage restrictions in patients or severe adverse effects. Therefore, the development of an easy-to-use agent for the treatment of hyperuricemia or the like has been desired.
Uric acid is eliminated mainly by the kidney, and the urate dynamics in the kidney has been investigated so far in some experiments using brush-border membrane vesicles (BBMV) prepared from the renal cortex (for example, see Non-patent references 7 and 8). It has been known that in human, uric acid is passed through the kidney glomerulus freely, and there are mechanisms of reabsorption and secretion of uric acid in the proximal tubule (for example, see Non-patent reference 9).
In recent years, the gene (SLC22A12) encoding the human kidney urate transporter has been identified (for example, see Non-patent reference 10). The transporter encoded by this gene (urate transporter 1, hereinafter referred to as “URAT1”) is a 12-transmembrane type molecule belonging to OAT family. URAT1 mRNA was specifically expressed in the kidney, and localization of URAT1 in apical side of the proximal tubule was observed on the human kidney tissue section. In an experiment using xenopus oocyte expression system, uptake of uric acid through URAT1 was shown. Furthermore, it was shown that the uptake of uric acid is transported by exchange with organic anions such as lactic acid, pyrazinecarboxylic acid (PZA), nicotinic acid and the like, and the uric acid uptake through URAT1 is inhibited by uricosuric drugs, probenecid and benzbromaronc. Thus, as expected by the experiment using membrane vesicles, it was strongly suggested that URAT1 is a urate/anion exchanger. That is, it was shown that URAT1 is a transporter that plays an important role in uric acid reabsorption in the kidney (for example, see Non-patent reference 10).
In addition, the relation between URAT1 and diseases became clear. Idiopathic renal hypouricemia is a disease wherein uric acid excretion is increased due to abnormal urate dynamics in the kidney and the serum uric acid level becomes low. It is known that the disease is often associated with urinary calculi or acute renal failure after exercise. URAT1 was identified as a causative gene of the renal hypouricemia (for example, see Non-patent reference 10). These things also strongly suggest that URAT1 is responsible for controlling the serum uric acid level.
Therefore, a substance having a URAT1 inhibitory activity is useful as an agent for the treatment and prevention of diseases associated with high serum uric acid levels, that is, hyperuricemia, gouty tophus, gouty arthritis, renal disorder associated with hyperuricemia, urinary calculi or the like.
In the treatment of hyperuricemia, it was reported that a combination of allopurinol of a uric acid production inhibitor and an agent having a uricosuric activity lowered the serum uric acid level more strongly than the single use of allopurinol (for example, see Non-patent references 11 and 12). Therefore, when treatment with an existing single agent can not exert effect enough, a higher therapeutic effect can be expected by a combination use of a uric acid production inhibitor and a uricosuric agent. Furthermore, for hyperuricemia with the uric acid underexcretion type, it is considered that since urinary excretion of uric acid can be decreased by lowering serum uric acid level, the risk of urinary calculi caused by the monotherapy with a uricosuric agent can be reduced. In addition, for hyperuricemia with the mixed type, high therapeutic effect is expected. Thus, an agent having both an inhibitory activity of uric acid production and a uricosuric activity is expected to become an extremely useful agent for the prevention or treatment of hyperuricemia or the like.
As a compound having both xanthine oxidase inhibitory activity and URAT1 inhibitory activity, morin, a natural product, is known (see Non-patent reference 13).
Benzoic acid or salicylic acid derivatives having xanthine oxidase inhibitory activity are known (see Patent references 1-5). However, in the references, anything is neither described nor suggested about indolizine derivatives of the present invention.    Patent reference 1: International Publication No. WO2007/043400 pamphlet    Patent reference 2: International Publication No. WO2007/043401 pamphlet    Patent reference 3: International Publication No. WO2008/126898 pamphlet    Patent reference 4: International Publication No. WO2008/126899 pamphlet    Patent reference 5: International Publication No. WO2008/126901 pamphlet    Non-patent reference 1: Atsuo Taniguchi and 1 person, Modern Physician, 2004, Vol.24, No.8, pp.1309-1312    Non-patent reference 2: Kazuhide Ogino and 2 persons, Nippon Rinsho (Japan Clinical), 2003, Vol.61, Extra edition 1, pp.197-201    Non-patent reference 3: Hideki Horiuchi and 6 persons, Life Science, 2000, Vol.66, No.21, pp.2051-2070    Non-patent reference 4: Hisashi Yamanaka and 2 persons, Konyosankessyo to Tsufu (Hyperuricemia and gout), issued by Medical Review Co., 1994, Vol.2, No.1, pp.103-111    Non-patent reference 5: Robert A Terkeltaub, N. Engl. J. Med., 2003, Vol.349, pp.1647-1655    Non-patent reference 6: Ming-Han H. Lee and 3 persons, Drug. Safety, 2008, Vol.31, pp.643-665    Non-patent reference 7: Francoise Roch-Ramel and 2 persons, Am. J. Physiol., 1994, Vol.266 (Renal Fluid Electrolyte Physiol., Vol.35), F797-F805    Non-patent reference 8: Francoise Roch-Ramel and 2 persons, J. Pharmacol. Exp. Ther., 1997, Vol.280, pp.839-845    Non-patent reference 9: Gim Gee Teng and 2 persons, Drugs, 2006, Vol.66, pp.1547-1563    Non-patent reference 10: Atsushi Enomoto and 18 persons, Nature, 2002, Vol.417, pp.447-452    Non-patent reference 11: S Takahashi and 5 persons, Ann. Rheum. Dis., 2003, Vol.62, pp.572-575    Non-patent reference 12: M. D. Feher and 4 persons, Rheumatology, 2003, Vol.42, pp.321-325    Non-patent reference 13: Zhifeng Yu and 2 persons, J. Pharmacol. Exp. Ther., 2006, Vol.316, pp.169-175